Glutamate-gated chloride channels, or H-receptors, have been identified in arthropod nerve and muscle (Lingle et al, 1981, Brain Res. 212: 481-488; Horseman et al., 1988, Neurosci. Lett. 85: 65-70; Wafford and Sattelle, 1989, J. Exp. Bio. 144: 449-462; Lea and Usherwood, 1973, Comp. Gen. Pharmacol. 4: 333-350; and Cull-Candy, 1976, J. Physiol. 255: 449-464).
Invertebrate glutamate-gated chloride channels are important targets for the widely used avermectin class of anthelmintic and insecticidal compounds. The avermectins are a family of macrocyclic lactones originally isolated from the actinomycete Streptomyces avermitilis. The semisynthetic avermectin derivative, ivermectin (22,23-dihydro-avermectin B1a), is used throughout the world to treat parasitic helminths and insect pests of man and animals. The avermectins remain the most potent broad spectrum endectocides exhibiting low toxicity to the host. After many years of use in the field, there remains little resistance to avermectin in the insect population. The combination of good therapeutic index and low resistance strongly suggests that the ligand-gated ion channels, and especially glutamate-gated chloride (LGIC/GluCl) channels remain good targets for insecticide development.
Glutamate-gated chloride channels have been cloned from the soil nematode Caenorhabditis elegans (Cully et al., 1994, Nature 371: 707-711; see also U.S. Pat. No. 5,527,703 and Arena et al., 1992, Molecular Brain Research. 15: 339-348) and Ctenocephalides felis (flea; see WO 99/07828).
In addition, a gene encoding a glutamate-gated chloride channel from Drosophila melanogaster was previously identified (Cully et al., 1996, J. Biol. Chem. 271: 20187-20191; see also U.S. Pat. No. 5,693,492).
Dermacentor variabilis (American dog tick) is indigenous to the majority of the U.S. with known common hosts of livestock, deer, dogs, humans and small mammals. This tick is associated with various diseases, including Rocky Mountain spotted fever, babesiosis, tick paralysis, anaplasmosis, tularemia and cytauxzoonosis.
Despite the identification of the aforementioned cDNA clones encoding non-tick LGIC/GluCl channels, it would be advantageous to identify additional genes which encode D. variabilis LGIC/GluCl channels in order to allow for improved screening to identify novel LGIC/GluCl channel modulators that may have insecticidal, acaricidal, and/or nematocidal activity for animal health, especially as related to treatment of tick infestations in livestock and domesticated animals, such as dogs and cats. The present invention addresses and meets these needs by disclosing novel genes which encode D. variabilis LGIC/GluCl proteins and when expressed in Xenopus oocytes result in formation of functional LGIC/GluCl channels.
Heterologous expression of a LGIC/GluCl channel of the present invention will allow the pharmacological analysis of compounds active against parasitic invertebrate species relevant to animal and human health, especially in the treatment of tick infestations directly related to Dermacentor variabilis. Heterologous cell lines expressing an active LGIC/GluCl channel can be used to establish functional or binding assays to identify novel LGIC/GluCl channel modulators that may be useful in control of the aforementioned species groups.